1. Field of the Invention
This invention relates to a chrome-free corrosion inhibitor composition, more particularly to a chrome-free corrosion inhibitor composition including a film-forming enhancer.
2. Description of the Related Art
A metal substrate usually has a protective coating thereon for providing corrosion resistance and for providing a shiny appearance to enhance for commercial appeal.
Conventional protective coatings have a composition including hexavalent chrome ion and have advantages of excellent corrosion resistance, self-repairability, shiny appearance, and low cost. However, hexavalent chrome is toxic and hazardous. Therefore, its use is restricted based on the Restriction of Hazardous Substance (RoHS) Directive, which is adopted by the European union and which restricts the use of specific hazardous materials in electronic and electrical products.
Moreover, since trivalent chrome can be converted into hexavalent chrome, potential hazards resulting from the use of trivalent chrome cannot be neglected. Therefore, there is a need to find a chrome-free corrosion inhibitor composition to replace the chrome-containing corrosion inhibitor composition.
Although conventional chrome-free corrosion inhibitor compositions, such as those disclosed in U.S. Pat. Nos. 5,380,374, 6,038,309, 6,059,867, 5,951,747, 5,728,233, 5,584,946, and 6,206,982, can provide corrosion resistance to metal substrates, they fail to maintain the shiny appearance provided by a decorative plating layer, such as Zn or Zn alloy, on the metal substrate.
U.S. Pat. Nos. 6,217,674, 5,449,414, and 5,432,456 disclose chrome-free corrosion inhibitor compositions that can provide corrosion resistance to a metal substrate. However, use of these compositions results in a protective film having a non-uniform appearance on the metal substrate.
U.S. Pat. Nos. 5,938,861 and 5,743,971 disclose chrome-free corrosion inhibitor compositions containing silicon compounds. These compositions are relatively unstable and can generate undesired silicate and silica precipitates when stored for a period of time.
U.S. Pat. No. 6,524,401 B1 discloses a chrome-free composition including titanium trichloride, an oxidant, such as organic acids, a complexing agent, and a metal compound. In use, the chrome-free composition is formed into a solution for application to a metal substrate. The solution is subsequently dried to form a protective coating on the metal substrate. During drying, titanium chloride is formed into titanate deposit deposited on the metal substrate to form the protective coating. The conventional chrome-free composition is disadvantageous in that the corrosion resistance, the compactness, the shiny appearance, and the uniformity of the protective coating formed therefrom still need to be improved. In addition, titanium trichloride is relatively expensive and unstable, i.e., it tends to form precipitates during storage. Moreover, the applicants conducted experiments and found that addition of the organic acids in the corrosion inhibitor composition improved insignificantly the corrosion resistance, the compactness, and the ability to maintain a shiny appearance for the protective coating formed therefrom.
Therefore, there is a need to provide a corrosion inhibitor composition that is low cost and environmental friendly, and that can provide excellent corrosion resistance, the ability to maintain a shiny appearance, and uniform appearance for the protective coating formed therefrom.